System design of an optical communication network is performed before operation of communication service, based on preset system conditions, such as fiber parameters, the signal modulation format, the bit rate, and the fiber input light power.
A system transmitting high-density wavelength division multiplex signal light requires prior design to deal with dynamic path switching in operation, wavelength increase, and change in the modulation method, in addition to consideration of penalty for interference between wavelength signals in transmission. However, in prior design, an excessive margin may cause deterioration in transmission distance and transmission quality of the whole system. For this reason, in the actual situation, there is a demand for operation of a system with a margin suppressed to a minimum and operation of a dynamic optical network.
For this reason, an error vector magnitude (EVM) is measured for each of optical transmission devices in the system, and an electrical signal to noise ratio (ESNR) is acquired from a measurement result. In addition, a method is well known in which an optical signal to noise ratio (OSNR) is calculated from the acquired ESNR, and an OSNR margin is calculated based on the OSNR. A conventional example is described in Japanese Laid-open Patent Publication No. 2015-50600.
However, the case of using an optical transmission device including no function of measuring EVM is also supposed, there are cases where measurement of EVM fails, which causes a problem. Accordingly, in the actual situation, a margin of the transmission quality is not accurately measured.